This invention relates to an improved thermal extraction system and method for underground heat sources and in particularly to thermal extraction from solid masses of crystal or rock salt or similar minerals.
It is known that highly heat conductive minerals such as rock salt and quartzite and the like sometimes occur in geological forms, such are known as spines or spires or domes or veins, and comprise relatively solid masses extended upward towards the earth's surface. These rock salt or mineral domes are within reach of modern drilling equipment.
The occurrence of this salt dome phenomena is fully described in U.S. Pat. Nos. 3,676,078 (Jacoby I) and 3,864,917 (Jacoby II) of Charles H. Jacoby and assigned to International Salt Company. In the background discussion, Jacoby I states that the expense and difficulties in attempting bore hole operations through ordinarily encountered geological structures to reach such depths as would encounter rock formations competent to support a constant and sufficiently high temperature heat extraction operation have proved prohibitive, and cites examples of such prior proposals in U.S. Pat. Nos. 2,461,449; 3,140,986; 3,274,769; and 3,363,664.
Jacoby I describes a method of solution mining and recrystalization recovery of solid sodium chloride from underground formations which utilize the heat conductivity of underground salt spires or domes or the like. Jacoby I penetrates a salt dome with a well and by utilizing water dissolves sufficient salt in the salt dome to form a cavern from which the brine is withdrawn. A second well is drilled for the purpose of solution mining salt. The first well is used as a heat source to heat a heat exchange fluid, such as any inert (to salt) gas or liquid. The heat exchange fluid is circulated from the heat reservoir through the evaporator and back to the heat reservoir which provide a source of heat for the evaporator. Brine from the solution mining well is pumped into the evaporator where the brine solution is vaporized in the evaporator with the salt being precipitated. The water vapor is condensed and returned to the salt cavern for further extraction of salt.
Jacoby I has several drawbacks, first at the depths where the relatively high temperatures exist, the rock salt will behave plastically. Unless a counter pressure is maintained in the cavern, the normal pressure, due to the weight of the salt and its overburden, is sufficient to cause the plastic salt to flow and thereby close the cavern (see U.S. Pat. No. 4,052,857, discussed later herein). Jacoby I also, requires mining, for each salt dome cavern in operation as a heat reservoir, of relatively large amounts of salt.
Jacoby II discloses a geothermal energy system, wherein a rock salt deposit or dome is penetrated by a bore hole to a suitable depth and the deposit is solution mined of the salt to provide a cavern of prescribed shape and dimension in the salt mass as a heat reservoir for the system. The heat reservoir is emptied of the salt solution and flushed with heat exchange fluid.
The heat reservoir of Jacoby II must be maintained as a highly controlled volume heat reservoir and the flow rate of the heat exchange fluid must be closely monitored to maintain a desired temperature. A pump is utilized to maintain the flow of the heat exchange fluid through the heat reservoir and the flow rate and/or temperatures are monitored to maintain the desired temperature.
Further, Jacoby II discloses a self moving heat exchange system with the hot heat exchange fluid withdrawn near the top of the cavern and the relatively cold fluid returned near the bottom of the cavern. Because the return fluid is cold having its heat extracted in a heat exchanger it is heavier than the relatively hotter heat exchange fluid. The cold heat exchange fluid being returned near the bottom of the cavern will cause the hotter fluid near the top of the cavern to rise and flow up the tubing and through the surface heat exchange device where its heat is extracted and then is returned as relatively cold fluid to the bottom of the cavern. Jacoby II restricts the heat exchange fluid as inert to the heat reservoir, since it must maintain the precise volume to control the temperature of the heat exchange fluid. The same plasticity problem encountered by Jacoby I is also present in Jacoby II.
Another Patent, U.S. Pat. No. 3,348,883, of Charles H. Jacoby, assigned to International Salt Company (Jacoby III), describes a method of mining and beneficiation of salt from a salt dome in which he discusses conveying the brine from a brine cavern in a salt dome in heat-insulated form upwardly to the point of discharge of the brine into an evaporator. Jacoby III suggests a suitable arrangement for insulating the brine upflow, is by covering the production tubing with heat insulating material or enclosing the production tubing within a larger conduit and suggest filling this conduit with dead air or vacuum. Of course, the conduit must be sealed to the brine production tubing to avoid contact with the brine in the brine cavern. Such arrangements are subject to various casualties such as leakage of seals for the dead air or vacuum, as well as, saturation of the insulating material with brine.
In another context a heat pump using a deep well for a heat source is disclosed in U.S. Pat. No. 2,461,449 of Marvin Smith, assignor to Muncie Gear Works, Inc. Smith describes the use of a conduit around a production tubing from the deep well to provide a surrounding airspace throughout its length. This casing is closed at each end and is secured to the production conduit by welding or another suitable manner. This arrangement suffers the same casualties as Jacoby III.
U.S. Pat. No. 4,052,857, assigned to Dow Chemical Company (Dow) describes a geothermal energy extraction process utilizing a tubing or pipe closed at one end and preferably pointed, weighted with removable weights, and sunk into the salt dome at a depth where the salt exhibits plasticity. Once the first pipe is installed and the weights removed a second open-ended, insulated pipe is inserted into the first pipe to provide a double heat exchanger. In this operation thermal energy is extracted from the salt by passing a heat exchange fluid either down the second pipe and up through the annulus between the first and second pipe or vice versa. Dow encounters limitation in sinking the closed tubing in the plastic salt because to overcome the salt density Dow uses uranium dioxide to obtain sufficient weight to overcome this high density. Dow process is further limited due to overburden between the top of the salt dome and the surface of the earth which dissipate heat from the well. Special note is made in the Dow Patent that the thermal conductivity of the overburden to that of the rock salt is inversely related to the insulating properties of the overburden, and suggests that an anhydrite caprock above a salt dome possesses the best insulating property.
U.S. Pat. No. 4,512,156 of Nagase discloses an apparatus and method for using terrestrial heat to increase the temperature of a liquid which comprises a pipe buried in the earth in the region of high subterranean heat with a second pipe telescoped therein. The first pipe has a digging head on it's lower end and the second pipe is open at its lower end to communicate with the first pipe. The pipes are insulated with thermal insulation material on the outer surface of the inner and outer pipes. Water or other heat exchange fluid is pumped down the annulus between the pipes and withdrawn through the inner pipe for transfer to surface heat exchangers for use as a source of heat energy.
U.S. Pat. No. 3,862,545 of Ellis discloses a process for using energy from a hot brine well to operate a steam turbine for electric power generation. Ellis utilizes brine from a geothermal well as a source for recovering thermal energy by vaporizing the hot brine and using the steam generated to operate a turbine.